1. Field of the Invention
The present invention relates to a load control system comprising a plurality of load control devices for controlling the amount of power delivered to a plurality of electrical loads from an alternating-current (AC) power source, and more particularly, to a lighting control system, which has sensors for detecting the presence and absence of an occupant and automatically changes between two modes of operation.
2. Description of the Related Art
Typical prior art load control systems are operable to control the amount of power delivered to an electrical load, such as, a lighting load or a motor load, from an alternating-current (AC) power source. A lighting control system generally comprises a plurality of control devices coupled to a communication link to allow for communication between the control devices. The control devices of a lighting control system include lighting control devices (e.g., dimmer circuits, electronic ballasts, or light-emitting diode drivers) operable to control the amount of power delivered to the lighting loads (and thus, the intensity of the lighting loads) in response to digital messages received across the communication link. In addition, the control devices of a lighting control system often include one or more keypad devices that transmit commands across the communication link in order to control the loads coupled to the lighting control devices.
In order to conserve energy, some prior art lighting control systems for commercial buildings provide an “afterhours” mode, which may be activated during hours when a building is typically not occupied, for example, during “non-work” hours, such as, from 6 P.M. to 7 A.M. When operating in the afterhours mode, the lighting control system turns off the lighting loads in most workspace areas of the building to save energy. Only a minimum number of lighting loads (such as, emergency or safety lighting) is typically illuminated in the building during afterhours mode. If an employee needs to occupy one or more of the workspace areas, the employee may use a manual control, such as, actuating a button of a keypad device, to turn on all of the lights in the workspace area. To ensure that the lights do not stay on for longer than required, the lighting control system turns the lights off (except for the emergency or safety lighting) after a predetermined amount of time from when the lights were turned on (e.g., one hour). Before turning off the lights (e.g. a short period of time before turning off the lights, such as, five minutes), the lighting control system blinks the lights in the workspace area to alert the employee that the lights will be turning off soon. The employee may then manually actuate a button of the keypad to cause the lights to stay on for another hour. The requirement to manually actuate the buttons of the keypad to keep the lights on is undesirable since this action may be disruptive to the employee.
To provide energy savings, some lighting control systems have included occupancy or vacancy sensors, which are used to detect occupancy or vacancy conditions in the area around the sensor to thus control the lighting loads of the lighting control system. An occupancy sensor typically operates to turn on lighting loads when the occupancy sensor detects the presence of an occupant in the space (i.e., an occupancy event) and then to turn off lighting loads when the occupancy sensor detects that the occupant has left the space (i.e., a vacancy event). A vacancy sensor only operates to turn off lighting loads when the vacancy sensor detects a vacancy condition in the space. Therefore, when using a vacancy sensor, the lighting loads must be turned on manually (e.g., in response to an actuation of a button of a keypad). In prior art lighting control systems, each sensor has been configured as either an occupancy sensor or a vacancy sensor. Typically, the sensors are configured as occupancy sensors during normal work hours (i.e., when the lighting control system is not in the afterhours mode), such that the lights of the lighting control system are only turned on when the workspace areas are occupied. The occupancy sensors may be disabled in the afterhours mode, such that, for example, security guards do not inadvertently cause the lights (other than the emergency or safety lights) to be unnecessarily turned on after work hours.
Thus, there exists a need for a lighting control system having an afterhours mode in which occupancy or vacancy sensors do not need to be disabled and in which the occupant does not need to repetitively actuate a button of a keypad (e.g., once every hour) in order to keep the lights on.